The diminishing reserves of fossil fuels and the emission of harmful gases connected with their use have increased the interest in utilizing biological materials, especially from non-edible renewable resources for making liquid fuels capable of replacing fossil ones. Several prior art processes are known for producing liquid fuels from biological starting materials. However, effective utilization of materials and energy is crucially important for the economy of biomass-to-liquid fuels processes.
Biomass-to-liquid (BTL) facilities are typically composed of the following main units; modules for biomass pre-processing, gasification, synthesis gas purification, Fischer-Tropsch (FT) unit, wax cracking-isomerization, product separation, gas circulation and oxygen production.
Synthesis gas from both renewable and fossil sources has been used to produce liquid fuels by the Fischer-Tropsch synthesis. US 2007/0225383 discloses a process for converting biomass to synthesis gas and providing a Fischer-Tropsch reaction for reacting the gas into fuels and chemicals. The invention improves the energy balance of the reaction by utilizing the heat of the exothermic Fischer-Tropsch reaction in the endothermic gasification reaction.
It is well known that biomass-to-liquid is a combination of exothermic (e.g. Fischer-Tropsch, gasification) and endothermic (e.g. biomass drying, product separations, gas purification) and thermo neutral (e.g. air separation unit, ASU) processes. A considerable amount of electricity is consumed in biomass-to-liquid by pressure swings and pumping intermediates and process coolants. The energy inventory of the whole biomass-to-liquid, electricity included, is however positive.
Different types of integrated processes, resulting in improved efficiency and in some cases lower utility costs are known from prior art. U.S. Pat. No. 5,624,964 describes integration of a steam reforming unit and a cogeneration power plant where a portion of compressed air from the power plant gas turbine air compressor is introduced to a combustor-regenerator of the steam reforming unit and where hot flue gas from the combustor-regenerator is fed back to the combustor of the cogenerating gas turbine power plant mixed with the remainder of the compressed air. Steam reforming is inter alia an integral component in Fischer-Tropsh processes and is traditionally carried out in multitubular fixed bed reactors which are heated on the outside in a furnace, e.g. by burning fuel such as methane and propane to supply heat for the reaction. In a cogeneration power plant, power is generated by burning fuel gas at moderate pressures to produce hot, pressurized gases which are then expanded and cooled to produce power and steam.
US 2004/0055716 discloses synthesis gas production in combination with the production of pulp and paper where a recovery boiler for chemical recovery is replaced with a Black Liquor Gasification Combined Cycle (BLGCC) which increases the energy yield in the pulp mill and where the synthesis gas production is more energy efficient and the produced synthesis gas is more suitable for methanol production. The combination of the processes makes the use of lower grade energy resources, like forestry waste wood possible. U.S. Pat. No. 6,180,684 discloses an integrated plant for preparation of synthetic fuel from hydrocarbonous gas and production of mechanical and electrical power in a gas turbine where warm exhaust gas from the gas turbine is used to prewarm the starting material, mechanical or electrical power is used for operation of machinery in the integrated plant and power is moreover exported for other purposes.
Prior art also describes the integration of a biorefinery and a pulp and paper mill where the process steam and hot water from a Fischer-Tropsch process is used by a paper mill replacing steam and hot water made through the burning of natural gas. The tail gas of the Fischer-Tropsch process is used to replace natural gas in the lime kiln. Moreover electricity is exported from the biorefinery to the mill and the chemical recovery boiler can be eliminated by processing the organics in the black liquor to syngas in the biorefinery and by recovering the cooking chemicals for reuse by the pulp mill. The syngas is cleaned-up and fed to the gas-to-liquid plant (The integrated forest biorefinery: the pathway to our bio-future, E. J. Connor, International chemical conference: efficiency and energy management, Quebec City, Canada, 29 May-1 Jun. 2007, pp 323-327).
Moreover, integration of a biomass-fired synthesis-gas process with integrated pulp and paper mills has been discussed. A combined heat and motor fuel plant (CHMF) located at the pulp and paper mill site benefits from the existing biomass-supply infrastructure of a mill. The synthesis-gas process provides fuel gas, preferably off-gases to the mill for final superheating of recovery-boiler steam, for fuelling the lime kiln or for fuelling gas-fired paper dryers. Black liquor from the mill can be partly gasified and in that case the hydrogen sulphide entering the main synthesis-gas processing line with the black-liquor-derived gas is removed in the gas-conditioning step and absorbed into the white liquor. A CO2 rich stream from the synthesis-gas conditioning step is utilized as the acidulation agent in the lignin-removal process (Biomass conversions; Integrated Forest Biorefinery Concepts, K. Saviharju and P. McKeough, Pulp & Paper Conference 5-7 Jun. 2007, pp 5-10).
Moreover, different approaches for the use of synthesis gas, Fischer-Tropsch tail gases and/or methanol as peak-shaving fuel in power plants has been suggested in prior art. WO 2007/061616 describes multiple syngas compositions for variable coproduction of electrical power and chemicals where the volume and/or composition of the syngas required for each function may vary over time. WO 2007/076363 discloses integrating of a Fischer-Tropsch hydrocarbon production facility with an electrical power generating facility where the peak-load power demand can be met by reducing the temperature of the Fischer-Tropsch reactor thereby increasing the quantity of tail gases and using Fischer-Tropsch tail gases to fuel a gas turbine generator set, thus achieving constant flow rates in the synthesis gas generating units and the Fischer-Tropsch units. U.S. Pat. No. 4,946,477 discloses an improvement to the methanol production step within an integrated gasification combined cycle (IGCC) electric power plant process where the methanol is produced from CO-rich synthesis gas and is used for peak-shaving. The water-gas shift and the methanol synthesis reactions take place simultaneously in a liquid-phase methanol reactor.
However, although integration of biomass-to-liquid plants or parts thereof with other facilities has been suggested for energy and chemical recovery, there is still a need for more efficient biomass-to-liquid processes. There is also a need for further improving the integration of biomass-to-liquid process with other industrial processes.
The present invention relates to the synergies of common waste water treatment of a biomass-to-liquid plant and another industrial facility such as a forest industry process, a power plant, an incineration plant, metal works, petrochemical plant or oil refinery when a wide-ranging integration of the at least two plants and/or processes are performed. The preferred other industrial process is a process for producing pulp and/or paper.